1. Field of the Invention
The present invention relates to magnetic communication systems. More particularly, it relates to a magnetic communication system which eliminates nulls in a mutual inductance field through a combination of signals from multiple transducers.
2. Discussion of the Related Art
When using a telephone, continually holding the handset to one's ear can be awkward. Also, holding the telephone interferes with the use of both hands for other work while trying to talk. In particular, the use of cellular telephones, which has increased dramatically, can interfere with the user's proper operation of an automobile. Various techniques have been used to overcome these difficulties.
Speaker phones allow one to talk while roaming around a room and using one's hands. However, speaker volume can disturb others around the user. They also cannot be used in close proximity to other speaker phones due to interference. They have limited privacy since the speaker broadcasts the conversation to all within earshot. Typically, the user must speak more loudly than normal to have proper reception at the microphone. Also, they tend to have poor sound quality because the user is not near the microphone and acoustics in the room are poor.
Headsets have been another way to free up the hands of a telephone user. Typically, the headset includes an adjustable strap extending across the user's head to hold the headset in place, at least one headphone located by the user's ear, and a microphone which extends from the headset along and around the user's face to be positioned in front of the users mouth. The headset is attached by a wire to the telephone. Headsets have the disadvantages of being bulky and somewhat awkward to use. Although they permit hands free use of the telephone, the user has limited mobility due to the connecting wire.
Wireless headsets have also been developed which eliminate the connecting wire to the telephone. The wireless headset uses radio frequency (RF) technology or infrared technology for communicating between the headset and a base unit connected to the telephone. The need for communications circuitry and sufficient power to communicate with the base unit increases the bulk and weight of the headset. This increased weight can become tiresome for the user. One alternative has been to attach the headset by a wire to a transmitting unit worn on the belt of the user. As with wired headsets, the wire can become inconvenient and interfere with other actions by the user. Significant interference rejection circuitry is also needed when multiple wireless headsets are used in close proximity.
Magnetic induction fields can be used to provide a communication link between a base unit and a headset. However, magnetic induction fields suffer from signal nulls at certain positions and orientations between the transmitter and receiver. When performing magnetic communications, a specific position and orientation between the transmitter and receiver is typically required. With a single transducer at the transmitter and receiver, certain positions and orientations result in no signal being received due to nulls in the mutual inductance between the transducers. The signal can be recovered by reorienting one of the transducers. It is also possible to use multiple, orthogonally positioned coils at the transmitter or receiver so that at least one coil does not have a null. Different mechanisms have been used to select or combine outputs from the transducers in order to provide communications.
In U.S. Pat. No. 4,489,330, a four coil transducer receiver includes a mercury switch array for selecting a coil transducer. As the receiver is moved, the switch array activates to pick up the positive phase components from the coils. However, this system cannot compensate for changes in position and orientation of the transmitter, and, thus, requires a stationary transmitter. Also, the mercury switch array is large, costly, and sometimes unreliable. Furthermore, switching transients occur as different coils are selected, which causes degradation of the signal and possible loss of information.
In U.S. Pat. No. 4,967,695, a three axis magnetic induction system used as a proximity detector is described. In this system, the outputs of the three coils are combined to provide a single received signal. While this system eliminates switching transients, it has other deficiencies. Since the output signal reverses polarity when it is rotated 180 degrees, the summed signal can be zero in some situations. Thus, the nulls present in the single transducer system are merely repositioned. Furthermore, the simple summing of signals from all three transducers can increase noise levels. For a proximity detector, noise is not a significant concern because it is merely attempting to determine the existence of a signal. Much better signal to noise ratios are needed in order to receive communication signals.